Application of Response Surface Methodology for the Extraction of Phytochemicals from Upcycled Kale (Brassica oleracea var. acephala)

Kale (Brassica oleracea) is recognized as a ‘superfood’ among leafy vegetables due to its high carotenoid content and potential health benefits. This study aims to optimize ultrasound-assisted extraction (UAE) to enhance the recovery of carotenoids and other phytochemicals from upcycled kale using response surface methodology. The optimized extraction parameters for carotenoids, i.e., aqueous ethanol as solvent, temperature, and extraction time at a fixed solid-to-solvent ratio, were established using the central composite design. The optimized extraction method was compared with other reported extraction methods for total phenolic content (TPC) and total antioxidant capacity (ferric reducing antioxidant power (FRAP) and 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging). Ultra-high-performance liquid chromatography–electrospray ionization–mass spectrometry (UPLC-ESI-MS) analysis was also performed. Under the optimized UAE conditions of 100% ethanol at 57 °C for 30 min, total carotenoid content of 392 µg/g dry weight (DW) was observed, though the predicted carotenoid content was 550 µg/g DW. Under these conditions, TPC, FRAP, and DPPH were 10.5 mg gallic acid equivalents/g DW, 13.9 µmole Trolox equivalence/g DW, and DPPH radical scavenging activity as IC50 of 2.04 ± 0.31 mg/mL, respectively. The UPLC-ESI-MS analysis showed the highest total phytochemicals recovered through microwave-assisted extraction, followed by UAE, compared to other tested extraction methods. In conclusion, the established optimized UAE process significantly enhanced the yield and quality of recovered phytochemicals from upcycled kale.